1. Field of the Invention
The present invention relates to a method of manufacturing a thin film transistor suitable for applications such as a switching device for displaying a pixel in a display panel of an active matrix type.
2. Description of the Related Art
FIG. 1 is a cross sectional view showing a structure of a thin film transistor of a bottom gate type.
This thin film transistor is formed as follows.
On a surface of an insulating transparent substrate 1, a gate electrode 2 of high melting point metal (refractory metal) such as tungsten or chromium is provided. The gate electrode 2 has a tapered shape with its width gradually increasing toward the transparent substrate 1. On the transparent substrate 1 on which the gate electrode 2 is located, a silicon oxide film 4 is deposited, with a silicon nitride film 3 interposed therebetween. The silicon nitride film 3 prevents penetration of impurities contained in the substrate 1 into an active region which will be described below, and the silicon oxide film 4 acts as a gate insulating film. A polycrystalline silicon film 5 is deposited on the silicon oxide film 4 crossing over the gate electrode 2. The polycrystalline silicon film 5 functions as an active region of the thin film transistor.
A stopper 6 of insulating material such as silicon oxide is disposed on the polycrystalline silicon film 5. The region of the film 5 covered with the stopper acts as a channel region 5c, while the remaining regions of the polycrystalline silicon film 5 act as a source region 5s and a drain region 5d. A silicon oxide film 7 and a silicon nitride film 8 are deposited on the polycrystalline silicon film 5 including the stopper 6 formed thereon. These silicon oxide and nitride films 7 and 8, respectively, are interlayer insulating films protecting the polycrystalline silicon film 5 including the source and drain regions 5s and 5d. 
Contact holes 9 are formed at prescribed positions of the silicon oxide and nitride films 7 and 8 located on the source and drain regions 5s and 5d. At the contact hole 9 portions, a source electrode 10s and a drain electrode 10d are disposed connected to the source and drain regions 5s and 5d, respectively. An acrylic resin layer 11 transparent to visible light is deposited on the silicon nitride film 8 where source and drain electrodes 10s and 10d are formed. The acrylic resin layer 11 fills in the recesses created by the gate electrode 2 and the stopper 6, to thereby planarize the surface.
A contact hole 12 is formed in the portion of the acrylic resin layer 11 located on the source electrode 10s. A transparent electrode 13 of indium tin oxide (ITO) or the like connected to the source electrode 10s through the contact hole 12 is formed spreading over the acrylic resin layer 11. This transparent electrode 13 forms a pixel electrode in the application of a liquid crystal display panel.
A plurality of the above-described thin film transistors and pixel electrodes are arranged in a matrix on the transparent substrate 1. These transistors respond to a scan control signal applied to the gate electrode 2 to provide image data received at the drain electrode 10d to the pixel electrodes.
The polycrystalline silicon film 5 is preferably formed to have a grain size large enough to function as an active region of the thin film transistor. One of the commonly-known methods for forming the polycrystalline silicon film with a large grain is a laser annealing method utilizing an excimer laser. According to this laser annealing method, amorphous silicon is deposited on the silicon oxide film 4 serving as a gate insulating film, and is irradiated by an excimer laser, so that silicon is melted and then crystallized. Since a use of the laser annealing method can eliminate the need for processing the transparent substrate 1 at a high temperature, a low-melting point glass substrate can be employed as the transparent substrate 1.
The amorphous silicon film, which will be turned into the polycrystalline silicon film 5, is formed by plasma CVD performed at a low temperature. As a result, hydrogen contained in silane which is used as reaction gas remains in the film. If the amorphous silicon film is melted by laser irradiation with a large amount of hydrogen remaining in the film, hydrogen is suddenly expelled from the film, resulting in a rough film surface. Consequently, operation characteristics of a transistor having such a polycrystalline silicon film as an active region will be deteriorated.
Therefore, an object of the present invention is to avoid creation of a rough film surface during crystallization of an amorphous silicon film.
A method of manufacturing a thin film transistor according to the present invention includes a first step of depositing a refractory metal film on a main surface of a substrate and etching the film to a prescribed pattern to form a gate electrode, a second step of depositing a gate insulating film on the substrate to cover the gate electrode, a third step of depositing a semiconductor film on the gate insulating film, and a fourth step of depositing an interlayer insulating film on the semiconductor film. The above third step includes a step of depositing an amorphous silicon film on the gate insulating film, heating the amorphous silicon film at 430xc2x120xc2x0 C. to expel hydrogen contained therein, and melting the amorphous silicon film to crystallize.
Another method of manufacturing a thin film transistor according to the present invention includes a first step of depositing a semiconductor film on a main surface of a substrate, a second step of depositing a gate insulating film on the semiconductor film, a third step of depositing an electrically conductive film on the gate insulating film and etching the electrically conductive film to a prescribed pattern crossing over the semiconductor film to form a gate electrode, and a fourth step of depositing an interlayer insulating film on the semiconductor film to cover the gate electrode. The first step includes a step of depositing an amorphous silicon film on the main surface of said substrate, heating the silicon film at 430xc2x120xc2x0 C. to expel hydrogen contained therein, and melting the amorphous silicon film to crystallize it.
In the above method of manufacturing a thin film transistor according to the present invention, the above third or first step of heating the amorphous silicon film is carried out in an inert gas atmosphere. Gas such as nitrogen gas can be used as the inert gas.
In the above heating step of the present invention, for example, prior to heating, the substrate is placed in a preparation chamber disposed adjacent to a heat treatment chamber, gas in the heat treatment and preparation chambers are replaced by the inert gas, and the substrate is relocated from the preparation chamber to the heat treatment chamber.
Further, according to the present invention, the step of heating the amorphous silicon layer is carried out for one hour or longer.
The step of heating the amorphous silicon film according to the present invention allows reduction in hydrogen concentration of the amorphous silicon film formed on the gate insulating film to 1 atomic % or lower.
By thus performing heat treatment of the amorphous silicon film, hydrogen remaining in the film when it is formed can be gradually expelled out of the film. Performing heat treatment for a given period of time allows reduction in hydrogen in the amorphous silicon film to a prescribed value or lower, preventing a large amount of hydrogen from being suddenly emitted from the film when the amorphous silicon film is melted.
Therefore, the present invention can avoid roughness at the surface of the film which otherwise results from sudden generation of hydrogen when the amorphous silicon film is melted.
Since heat treatment for eliminating hydrogen is carried out in an nitrogen atmosphere, it is possible to avoid entry of impurity ions into the amorphous silicon film. As a result, operation characteristics of a thin film transistor having the polycrystalline silicon film as an active region can be improved.